1. Field of the Invention
The present invention relates to the field of electronic equipment for attachment to a telephone line, and more particularly, to electronic equipment of the stated type which includes a phone line ring signal detection capability to initiate an auto answer function therein.
2. Prior Art
Electronic equipment in the nature of communication devices which attach to a telephone line and which incorporate an auto answer function is well known in the prior art. Such equipment may include, by way of example, telephone answering machines which sense the presence of a ring signal on a telephone line and, after a predetermined number of rings, answer the phone with a prerecorded voice message, after which a message from the calling person is recorded. Equipment of this general type also includes modems (modulator/demodulator sets for digital communication over phone lines) which have an auto answer function, enabling one digital device to call another digital device over the phone line for data communication therebetween. In any equipment of this general type, some provision must be made for detecting the presence of the ring signal on the phone line, as the auto answer function may only be initiated thereby. In the case of a modem, the auto answer function is normally immediately initiated on the first ring signal, once it has been effectively determined that a ring signal rather than a line transient is present.
The problem of detecting a ring signal on telephone line is complicated by the characteristics of a telephone line and by governmental requirements for equipment to connect thereto. In particular, in the United States, the open line or on hook line voltage is approximately 50 volts DC, high for the direct connection of low cost electronic devices therebetween. Further, in accordance with the applicable requirements, the ring detect circuit must be isolated from local ground so that isolation must be provided between the circuit which senses the AC ring signal between the tip and ring lines and the circuit which processes the AC signal to identify it as a ring signal. Consequently, in the prior art, the most common ring detect circuit is comprised of an opto coupler having the emitting device therein capacitively coupled between the tip and ring lines and the receiving device utilized as part of an active circuit to detect the AC signal. Such circuits work very well, providing good ring signal detection and excellent line isolation. However, while they are not particularly large or expensive in comparison to most prior art modems and similar equipment, they tend to be limited in size and cost reduction possible and not readily subject to realization by way of integrated circuits and accordingly, are less than ideal for use as higher and higher integration of modem circuitry naturally progresses.